Add Logical Session & Transaction Management

[niclaflamme]

This PR introduces two new modules to PgDog’s frontend: logical session management and logical transaction management. Both aim to enforce correct behavior across a sharded backend without leaking complexity to clients.

LogicalSession

• New struct LogicalSession<'a> tracks all SET-style session variables.

• Clears and re-fans out variable settings to relevant shards on change.

• APIs:

  • set_variable(name, value) – record a new setting and clear shard sync state.
  • get_variable(name) – fetch current value, if any.
  • sync_shard(shard) / is_shard_synced(shard) – mark/check shard as up-to-date.
  • store_swapped_value / take_swapped_value – stash and retrieve prior shard values.
  • reset / reset_after_take – clear state, returning any swapped values.

• Complete unit tests covering all methods and edge cases.

LogicalTransaction

• New struct LogicalTransaction enforces one-shard-per-transaction discipline.

• Tracks lifecycle: Idle → BeginPending → InProgress → (Committed/RolledBack).

• Shard routing:

  • soft_begin() – start a pending transaction.
  • execute_query(shard) – touch a shard (first statement flips to InProgress).
  • commit() / rollback() – finalize or abort.
  • set_manual_shard(shard) – pin transaction to a specific Shard::Direct(n).
  • reset() – clear all state safely at any point.

• Errors enforce invalid transitions and multi-shard conflicts.

• Exhaustive tests for all states, error paths, and shard-conflict rules.

Future improvements

• Track per-variable synced shards to skip redundant SET fan-outs.

Todo

• Replace client.in_transaction (bool) with client.transaction (LogicalTransaction) and follow the compiler.
• in_transaction can be inferred.